Normally, modern digital mobile radio systems are exclusively full-duplex systems. Full-duplex systems are distinguished in that it is possible to send and receive data simultaneously. Reference is also made to full-duplex systems in this context when sending and receiving are not performed simultaneously but changeover between the transmission directions takes place without being noticed by the subscriber.
In principle, a distinction is drawn between two duplex methods. In the case of “frequency division duplex” (FDD), sending and receiving are performed in different, dedicated frequency bands. In the case of time division duplex (TDD), on the other hand, the transmission directions are divided into different times or time slots.
In the widely used mobile radio standard GSM (Global System for Mobile Communication), TDD and FDD methods are used in combination in order to be able to provide transceivers which can be manufactured as inexpensively as possible and can be integrated as well as possible.
Particularly in the reception part of modern mobile radio systems, the “low-IF receiver architecture” is increasingly being used instead of the homodyne receiver architecture (direct conversion) on account of the offset problems that arise therein. In the case of the low-IF-receiver architecture, the radio-frequency signal received is first of all down-converted to a relatively low intermediate frequency (IF) and in a second step to baseband.
In transceivers designed in this manner between which there is a point-to-point connection, the problem regularly arises that the intermediate frequency differing from zero means that it is always necessary to change channels between the send and receive time slots. That is to say that the phase locked loop in the transceiver always needs to lock onto a new frequency between send and receive time slots, even when the nominal channel does not actually need to be changed. However, this disadvantageously results in the net data transmission rate achieved using the transmission channel being greatly reduced from the gross data rate.
The document DE 100 46 586 specifies systems for data transmission that solve this problem by setting the reception sideband of the low-IF-receiver, “sideband selection”. In this case, for a point-to-point connection, one of the two receivers down-converts a received radio-frequency signal to a complex-value intermediate-frequency signal with a positive intermediate frequency and the other receiver down-converts it to an IQ signal with a negative intermediate frequency. This practice means that the phase locked loop does not need to lock onto a new carrier frequency between every send and receive time slot and that a phase locked loop (PLL) with a slow locking time can therefore advantageously be used.
As an alternative to the use of positive or negative intermediate-frequency levels, a receiver structure with an image-frequency suppressing mixer may also be used in both radio parts of the point-to-point connection.
However, it remains a drawback of such receiver structures that the receivers need to be designed, in terms of the second down-conversion mixing stage, which down-converts from intermediate frequency to baseband, both for a situation in which the channel frequency is below the local oscillator frequency of the second mixing stage and for a situation in which the channel frequency is above the local oscillator frequency of the second mixing stage.
An at least theoretical possibility for solving this problem involves the corresponding analog and digital components being of duplicate design for the two regularly occurring cases described. This naturally means increased additional complexity.